That's a largely rhetorical question; I could imagine several reasons why the game isn't here. Perhaps the fact that it's by a dread western furry artist, or perhaps it's the online suggestion that hangs a goose.
My loose (very loose) understanding of the game is that it's a Furry Dark Souls, except without all the wallowing in misery, pity, and sadness.

Per a [YET UNNAMED] place, the descriptor of features:
"ATLYSS" is a (Single-player + Multi-player) Action Online Role Playing Game
Enriched, instanced maps to discover
Equipment, Trade Items and Consumables to collect
Many Character Customization options with 5 races to choose from
3 Primary Classes to become at lv-10 (Fighter, Bandit, Mystic)
Skill load-out system for making builds to suit your game-play style
Character stat progression / leveling systems / attributes
7 Weapon types to use in combat (Katars, Heavy Melee, Scepters, and more)
Repeatable Questing / Single Quests to complete
Aesthetics inspired by 6th Generation video games

There's a GOG wishlist entry, but the fool of a took forgot to fill it out with any hook or information.

Main reason is perhaps that it is just released in Early Access, and then would need to maintain an EA game on two different platforms.

Does the publisher even want to release the game here?

The game has online components and if they need a third party client like Galaxy it would get a lot of push back by the community as this would be seen as DRM.

Why isn't it here? Because it's early access and the dev is a smaller dev. No one wants to waste time keeping two versions of a product uptodate when it is still being made, so I'm guessing its an efficiency thing. The manpower that would be spent updating GOG is better spent improving the final product.

EDIT: They do have a name your price entry on itch.io: https://kiseff.itch.io/atlyss
And it does seem to be a solo developer.

It looks like the itch.io build is the single player demo build only. So basically, you can get the demo from there, and give the dev a tip if you like